swarm-ui: move create-agent page off /agents, rename to CreateAgentPage

mara: the route should reflect creating an agent, and stay separate
from a future agent list page. Renamed AgentsPage -> CreateAgentPage
(file, component, css classes) and moved the route from /agents to
/create-agent - flat, not /agents/new, since index.html's relative
asset links only resolve correctly one path segment deep (filed
separately as a real bug, not fixed here). Leaves the bare /agents
path free for a future roster page.

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -19,7 +19,7 @@ import './Shell.css';
 
 const NAV_ITEMS: { href: string; label: string }[] = [
   { href: '/', label: 'overview' },
-  { href: '/agents', label: 'agents' },
+  { href: '/create-agent', label: 'new agent' },
   { href: '/jobs', label: 'jobs' },
   { href: '/components', label: 'components' },
 ];